Coal ash, also referred to as coal combustion residuals or CCRs, is produced primarily from burning coal such as at coal-fired power plants. Coal ash includes a number of by-products produced from burning coal, including                Fly Ash: a fine, powdery material generally composed mostly of silica made from burning finely ground coal in a boiler.        Bottom Ash: a coarse, angular ash particle that is too large to be carried up into smoke stacks, so it forms in the bottom of the coal furnace.        Boiler Slag: molten bottom ash from slag tap and cyclone type furnaces that turns into pellets that have a smooth glassy appearance after cooled with water.        Flue Gas Desulphurization Material: material left over from the process of reducing sulfur-dioxide emissions from a coal-fired boiler that can be a wet sludge consisting of calcium sulfite or calcium sulfate or a dry powdered material that is a mixture of calcium sulfites and calcium sulfates.        
Other types of by-products from burning coal can include:                fluidized bed combustion ash,        cenospheres (lightweight, inert, hollow spheres made largely of silica and alumina and filled with air or inert gas, typically produced as a byproduct of coal combustion at thermal power plants), and        scrubber residues.        
Coal ash is one of the largest types of industrial waste generated in the United States. According to the American Coal Ash Association's Coal Combustion Product Production & Use Survey Report, nearly one hundred thirty million tons of coal ash were generated in 2014. See e.g., United States Environmental Protection Agency (US EPA), “Coal Ash Basics” (Feb. 5, 2019), retrievable from httwww.epa.gov/coalash/coal-ash-basic.
Coal ash is disposed of or used in different ways depending on the type of by-product, the processes at the power plant, and the regulations the power plant has to follow. Some power plants may dispose of coal ash in surface impoundments or in landfills. Others may discharge it into nearby waterways under the plant's water discharge permit.
Coal ash contains toxic metals and metalloids such as mercury, cadmium and arsenic. Without proper management, these contaminants can pollute waterways, ground water, and drinking water. The three metals, lead, mercury and cadmium, and the metalloid arsenic have all caused major human health problems in various parts of the world. The overt toxicity of these elements has been recognized for many years; indeed, the harmful effects of lead were known as far back as the second century BC in ancient Greece. Over the years, physicians became increasingly familiar with the symptoms of metal poisoning arising in occupationally exposed workers and in individual cases of poisoning. Exposure to such metals can cause grave health issues in both adults and children. See Hutton, “Human Health Concerns of Lead, Mercury, Cadmium and Arsenic”, Chapter 6, Lead, Mercury, Cadmium and Arsenic in the Environment (John Wiley & Sons 1987).
Large spills near Kingston, Tenn., and Eden, N.C., highlighted need for action to help ensure protective coal ash disposal. These spills caused widespread environmental and economic damage to nearby waterways and properties. In response, the United States Federal Government enacted rules to regulate the disposal of coal ash. See Federal Register 80 FR 21301 (Apr. 17, 2015). More recently, the Environmental Protection Agency has proposed allowing states and the EPA to incorporate flexibilities into their coal ash permit programs.
Waterway pollution resulting from coal ash is far from being solved, and represents a long-felt but unsolved need. What is needed: an efficient, cost-effective, efficacious technique for removing coal ash, coal ash components, and/or other pollutants from waterways, ponds, marshes, holding tanks and other water sources and supplies.
More broadly, the world needs better ways to extract and/or remove trace elements and/or other materials such as for example lead, mercury, cadmium and/or arsenic from liquids such as water.